Superimposition characteristics are important product characteristics for power choke coils used in DC/DC converters and other power supply circuit components.
Laminated power choke coils (laminated choke coils) adopt the method to form a nonmagnetic layer in a location where magnetic fluxes are concentrated, by means of simultaneous sintering with a magnetic layer, to suppress magnetic saturation and thereby improve superimposition characteristics.
Patent Literatures 1 and 2 describe examples of the above method, where a nonmagnetic layer is made of, for example, Zn—Cu ferrite whose component elements are close to Ni—Zn—Cu ferrite that constitutes a magnetic layer.
In Patent Literature 3, use as a nonmagnetic layer of a ceramic material selected from ZnFe2O4, TiO2, WO2, Ta2O5, cordierite ceramics, BaSnN ceramics and CaMgSiAlB ceramics is described.
However, Patent Literature 3 does not mention using Ni—Zn—Cu ferrite as a magnetic layer, and ZnFe2O4 (zinc ferrite) is the only specific example of a nonmagnetic layer given and there is no mention of TiO2 in particular. On the other hand, Patent Literature 4 describes “a dielectric ceramic composition produced by blending TiO2 with 0.1 to 10 percent by weight of ZrO2, 1.5 to 6.0 percent by weight of CuO, 0.2 to 20 percent by weight of Mn3O4, and 2.0 to 15 percent by weight of NiO, to a total percentage by weight of 100,” while Patent Literature 5 describes “a dielectric ceramic composition characterized in that it is constituted by CuO (1.0 to 5.0 percent by weight), Mn3O4 (0.2 to 10 percent by weight), NiO (0.5 to 14 percent by weight), Ag2O (0.1 to 10 percent by weight), and TiO2 making up the remainder.” However, each only indicates that such a composition can be used as a material for capacitors used in combined inductor/capacitor components, and its use for nonmagnetic layers in laminated inductors is not indicated.
As described in Patent Literatures 1 and 2, however, use of a nonmagnetic layer made of Zn—Cu ferrite results in the Zn component of Zn—Cu ferrite diffusing to Ni—Zn—Cu ferrite in the simultaneous sintering process, and the Ni component of Ni—Zn—Cu ferrite diffusing to Zn—Cu ferrite, thereby causing a formation of Ni—Zn—Cu ferrite whose Ni concentration has a slope. These diffusion layers are constituted by Ni—Zn—Cu ferrite whose Curie point differs along the Ni concentration slope, meaning that as the temperature rises, areas of low Ni concentrations change from magnetic to nonmagnetic. This has been a problem because the apparent nonmagnetic layer thickness changes with the temperature, resulting in poor temperature characteristics of the product.
Also, a laminated choke coil has a conductive layer formation region where conductive layers constituting a coil are laminated alternately with magnetic material layers with at least one nonmagnetic layer inserted therebetween, and a yoke region constituted by magnetic material layers that are positioned at the top and bottom in the direction of lamination and serve as a yoke to connect the magnetic fluxes formed on the inner side of the coil and magnetic fluxes formed on the outer side of the coil. Accordingly when a laminated choke coil is sintered, sintering progresses as the sintering of the metal constituting the coil-constituting conductive layers interacts with the sintering of the magnetic material constituting the magnetic material layers, in the conductive layer formation region constituting the coil. In the yoke region, on the other hand, sintering progresses mainly in the magnetic material, and accordingly latent stress tends to generate between the two regions. Therefore, the nonmagnetic layers which are located in the conductive layer formation region constituting the coil and which have low affinity with magnetic material layers and coil conductive layers become thresholds of latent stress relief, and for this reason delamination occurs easily between the nonmagnetic layers and the adjacent magnetic material layers or coil-constituting conductive layers.
In addition to Zn—Cu ferrite, glass materials are generally known as nonmagnetic materials. Since their coefficients of linear expansion are different from those of ferrites, simultaneous sintering of ferrite and glass materials will cause delamination at the bonded interface.
Also a TiO2 material sintered at low temperature is applied as a nonmagnetic material that can be simultaneously sintered with magnetic layers. However, this specification does not allow a sufficient inter-diffusion interface to form and sometimes separation occurs at the interfacial layer.
Patent Literature 1 Japanese Patent Laid-open No. Hei 11-97245
Patent Literature 2 Japanese Patent Laid-open No. 2001-44037
Patent Literature 3 Japanese Patent Laid-open No. Hei 11-97256
Patent Literature 4 Japanese Patent No. 2977632
Patent Literature 5 Examined Japanese Patent Laid-open No. Hei 8-8198